Diesel engines efficiently convert the latent heat of hydrocarbon fuel into useful mechanical power. In the operation of conventional diesel engines, a metered amount of fuel is injected into each cylinder of the engine at recurrent intervals synchronized with rotation of the engine crankshaft to coincide with the air-compression stroke of a reciprocating piston. As the pressure in the combustion chamber increases, the compression temperature in the cylinder rises and the injected fuel is soon hot enough to ignite. The resulting combustion or firing of fuel in the cylinder forces the piston to move in the opposite direction, thereby applying torque to the engine camkshaft.
Conventional diesel engine fuel is a relatively low grade, refined petroleum known generally as diesel fuel oil which has desirable ignition and heat release characteristics. Diesel fuel oil has acceptably low levels of corrosive, abrasive and other noxious matter, and it is in ample supply at the present time.
Diesel engines typically burn fuel in a diffusion combustion mode. In that mode, the fuel burns as it comes into the chamber before it is well mixed with air. Since only pure air is compressed in the compression stroke of the engine, a high compression ratio can be used to obtain high cycle efficiency. However, the local combustion air to fuel ratio cannot be controlled.
On the other hand, gasoline engines, such as automobile engines, burn fuel after it has been well mixed with air. This mode of burning fuel is called premixed combustion. In this mode, the fuel is pre-mixed with air to form a combustible mixture. The mixture is compressed in the compression stroke of the engine. The compression ratio is limited to a lower value to avoid the hazardous "detonation" or "knock". Lower cycle efficiency results. However, the combustion air to fuel ratio can be controlled. In this case, the advantage of "lean burn" can be obtained. The conventional gas engine operates in this combustion mode. Usually, the gas fuel is introduced at low pressure either into the intake manifold or directly into the engine cylinder before the compression stroke. This is also known as low pressure gas engine. There are other advantages and disadvantages to premixed combustion, which are well known to those skilled in the art. There are also other advantages and disadvantages of diffusion combustion, which are also well known to those skilled in the art.
Recently, some dual fuel gas engines, called high pressure injection gas engines, have become known in the art. They also utilize a diffusion combustion mode, with ignition source which is used to ignite the fuel.
More than 65 years ago it was recognized that a small amount of readily ignitable pilot fuel could be injected in diesel engines to improve combustion of "heavy" hydrocarbon fuels that are otherwise difficult to ignite. See British Patent No. 124,642. As used herein, the term "pilot fuel" means relatively light hydrocarbon fuel (e.g. methanol or even standard diesel fuel oil) characterized by being significantly easier to ignite than the primary fuel in the injection system.